The present invention relates to a process for reducing plutonium contained in a plutonium waste liquor resulting from a retreating process of a used nuclear fuel or a treating process of a retreated waste liquor or from a plutonium-processing plant, or plutonium used in those facilities.
As reducing agents for plutonium, there have been used uranous nitrate, ferrous sulfamate, ferrous nitrate and the like.
For example, in the chemical process for re-treating a used fuel, uranous nitrate is used as reducing agent at present. For this purpose, uranyl nitrate must be first reduced into uranous nitrate by electrolysis. Uranous ion reacts with nitric acid ion to form nitrous acid according to the following reaction formula (1): EQU U.sup.4+ +NO.sub.3.sup.- +H.sub.2 O.revreaction.UO.sub.2.sup.2+ +HNO.sub.2 +H.sup.+ ( 1)
This reaction proceeds very slowly. If the reaction system is left as it is, the nitrous acid thus formed oxidizes the uranous ion rapidly and continuously according to the following reaction formula (2): EQU U.sup.4+ +2HNO.sub.2 .fwdarw.UO.sub.2.sup.2+ +2H.sup.+ +2NO (2)
Therefore, hydrazine is incorporated previously in the reaction system as an antioxidant for the uranous ion. If hydrazine is incorporated in the reaction system, it reacts with nitrous acid to form decomposition products, i.e. N.sub.2 O and N.sub.2 gases according to the following reaction formulae (3) and (4): EQU N.sub.2 H.sub.5.sup.+ +HNO.sub.2 .fwdarw.HN.sub.3 +2H.sub.2 O+H.sup.+( 3) EQU HN.sub.3 +HNO.sub.2 .fwdarw.N.sub.2 O.uparw.+N.sub.2 .uparw.+H.sub.2 O (4)
Since the above decomposition reactions proceed very rapidly, nitrous acid formed slowly according to the reaction formula (1) is decomposed rapidly according to the reactions shown by the above-described reaction formulae (3) and (4) to prevent oxidation of uranous ion shown by the reaction formula (2). Such a prevention of oxidation can be observed also in the case of plutonium (III) reduced by uranous ion. In the case of plutonium, nitrous acid is formed slowly according to the reaction formula (5). The nitrous acid thus formed reacts rapidly with plutonium (III) to oxidize the latter according to the reaction formula (6): EQU 2Pu.sup.3+ +NO.sub.3.sup.- +3H.revreaction.2Pu.sup.4+ +HNO.sub.2 +H.sub.2 O (5) EQU Pu.sup.3+ +HNO.sub.2 +H.sup.+ .revreaction.Pu.sup.4+ +H.sub.2 O+NO (6)
In order to prevent the oxidation, hydrazine is used as in the case of uranous ion. Hydrazine decomposes nitrous acid according to the above-described reaction formulae (3) and (4) to prevent plutonium from re-oxidation. Either way, hydrazine has been used heretofore as a mere stabilizer for uranous ion or plutonium (III).
Uranium used as reducing agent for plutonium (IV) in this chemical re-treating process is a useful nuclear fuel substance. Uranium necessitated as reducing agent for plutonium is increased in amount as treating capacity of the plant is increased and, accordingly, the absolute quantity of uranium stocked in the plant is increased to cause a great economical loss in a nuclear fuel cycle. Further, a large amount of uranium will be required in future for treating a great amount of plutonium contained in a used fuel from fast breeders. Thus, plutonium treatment will become difficult according to the conventional process in the future.
Under the circumstances as mentioned above, investigations on electrolytic reduction of plutonium have been made recently in Karlsruhe Institute, West Germany. (H. Schmieder, Electrolytic methods for application in the Purex process, ISEC-74-3.) However, cost of this electrolytic process is high, since a special extractor comprising a platinum mixing pillar anode and a titanium alloy cathode is required in this process. In addition, this process includes difficulties such as that precise control of the electrolytic current is required in respective steps in the extractor. Also in this electrolytic process, hydrazine is used as a stabilizer for electrolytically reduced plutonium (III).
On the other hand, in the treatment of waste liquors from group separation of re-treated waste liquors or from plutonium-processing plants, ferrous sulfamate, ferrous nitrate, etc. are used as reducing agent for plutonium for the purpose of analysis. However, these reducing agents yield an increased amount of waste products, since they contain iron. Moreover, the iron acts as an impurity to make the analysis difficult. Particularly, in case ferrous sulfamate is used, another defect is caused in that metal materials used are corroded by sulfamic acid, in addition to said defects. Also in this process, hydrazine is used as stabilizer for plutonium (III).
Namely, in conventional processes as above-described, hydrazine incorporated in the reaction system is used merely as stabilizer for uranous ion or plutonium (III).